I hope this kicks in the logic thing...
Originally Posted by holmburgers
Colour slides are processed by reversal. So when chrome film is underexposed, less halide is developed during the first BW development stage, and leaves more undeveloped halide in the emulsion for the colour development after reversal.
When more halide is left for reversal, more gets to develop. And when more silver develops, the coupling process produces more dyes. And what happens with more dye? More saturation.
Colour negatives aren't developed by reversal. So underexposing a colour negative emulsion will not yield the same effect as a film which goes through two development, and two exposure stages as it would in reversal (E6) processing
Underexposure will expose less halide- when less develops, less dyes form too. With less dyes, no saturated colours. Just muddy ones- grey from the lack of exposure, with little dye forming.
Overexposing (slightly) a colour negative stock OTOH makes more halide develop-able. More halide developing means more dyes.
A colour negative with punchy dyes will print with more punch on a positive material, when optically printed, that is.